The overall purpose of this SDRC Pilot and Feasibility grant is to confirm and extend our preliminary finding that CHOP/gadd153 (or simply CHOP) regulates apoptosis in keratinocytes of the epidermis following UVB irradiation of the skin in vivo. CHOP is a transcription factor, induced by environmental agents that cause protein unfolding and membrane damage. One of these agents is UVB light. Our preliminary data shows that CHOP is increased in murine skin after UVB irradiation. Also, keratinocyte apoptosis (formation of sunburn cells) is increased in the epidermis of CHOP knockout mice. Aim 1 will search for molecular targets responsible for CHOP's effect upon apoptosis, using cultured keratinocytes and fibroblasts from CHOP knockout mice. We will systematically examine particular caspase enzyme pathway(s) that may be targets for CHOP during UVB-mediated apoptosis. We will ask whether enhancement of UVB-apoptosis is specific to keratinocytes, or occurs similarly in fibroblasts and keratinocytes. Finally, we will test the reversibility of any of the effects discovered in the cell culture studies by comparing results from CHOP-deficient cells to results obtained from keratinocytes in which CHOP is overexpressed from an adenoviral vector, or from a tetracycline-inducible promoter construct. Core B (Cell Culture and Molecular Technology) will be called upon for some of the studies in Aim 1. Aim 2 will ask whether mechanisms responsible for CHOP's effects on UVB-apoptosis in vitro are the same as those responsible for CHOP regulation of UVB-induced apoptosis in vivo. Mouse and human skin will be UVB-irradiated, and biopsied determine the time-course and dose-responsible for CHOP expression, caspase3 aactivation, and bcl-2 expression. For Aim 2, Core A (Cellular and Molecular Morphology) and Core C (Translational Research will be utilized.